Spectrophotometers are known that use a single light source, a single detector and dual beam fiber optics that split the light into a reference beam and a sample beam. Such devices are shown, for example, in U.S. Pat. No. 4,061,428 and an article by Konnerth et al. entitled "In-Situ Measurement", IEEE Transactions (7/75). A common feature of such previous devices is the use of a filter unit wheel having apertured portions to allow both the reference beam and the sample beam to pass through a selected filter in the aperture. However, considering the device of the '428 patent, it is noteworthy that the triggering apertures 33 in filter unit wheel 22 are identical in number to the filter apertures 24. That is, there are disclosed 3N apertures 24 and 3N apertures 33. Further, the angular placement of apertures 24 is the same as such placement of the bifurcated fiber optics 17 and 14. As a result, the only way to prevent both the sample beam and the reference beam from arriving at the detector simultaneously (an unacceptable result), is to position an oscillating shutter 46 adjacent to filter unit 22. (Although there are N more apertures 38 also in filter unit 22, these are used to detect red versus green versus blue filters, and do not control whether a reference beam or a sample beam is in place). Such a use of a filter unit and a shutter has a decided disadvantage--there are at least two moving parts required, each of which is subject to wear and risk of breakdown.
In the Konnerth article noted above, there appears to be only one moving part in the spectrophotometer of FIG. 1, in which case a triggering mechanism would not be used to inform the computer as to the location of the filter unit. A highly accurate motor would, however, be required since the timing of filter unit rotation would have to be exact. For such an arrangement, the costs are high. A more preferred system is one in which the positioning can have wide tolerances because the actual rather than theoretical positioning is detected.
More recent attempts to provide spectrophotometers have not solved the noted problems. U.S. Pat. No. 4,648,714 teaches a dual beam spectrophotometer having a single moving part (filter unit 58 with four filter apertures 17(a)-17(d)). However, it does this at the expense of two detectors for reference and sample (19 and 37). The use of two different detectors is well known to be a source of drift error.
U.S. Pat. No. 4,097,743 teaches an optical system for a moisture analyzer in which two light beams are reflected at locations 180 degrees apart on a rotary wheel. The wheel has two filters and a window, all equally spaced 120 degrees apart. A series of mirrors are attached to the wheel at fixed positions, and provide signals to a sensor indicating wheel positions. Outputs are produced by the analyzer in the form of two time related pulse trains: a train of spaced pulses representing individual signals from each beam-filter combination and a train of periodic commutator pulses identifying wheel positions including a home position. The momentary pulses are used by a decoder circuit to separate the other pulse train, after amplification, into separate signals representing the beam-filter combinations. At the home position, a portion of the detection circuit is grounded, apparently providing an electronic "dark" signal. The decoded signals are combined, along with a reference voltage to provide a voltage representing sample moisture content which is displayed. This device has the shortcoming that detector related "noise", such as dark current, is not corrected for, in that the electronic " dark" signal is exclusive of the detector, which is grounded, regardless of whether or not residual photocurrent is flowing.
Some previous Raman spectrometers have used filters to attempt to provide a simple instrument. U.S. Pat. No. 4,586,819 teaches the use of a filter wheel with a laser and a monochromator. The filter wheel is used at an angle to the excitation laser beam and to the returning scattered Raman and laser signals from the sample. The device uses a laser oscillator to change the wavelength of the excitation beam. The function of the filters is to allow the reflected laser beam to pass on to a camera, and also to reflect the Raman scatter into a monochromator. The monochromator is used to select the Raman frequencies. Therefore, the filter wheel functions as a variable blocking filter, preventing the laser beam from entering the monochromator. It does not modulate the signal, switch from sample to reference, or select the Raman frequency for analysis. The instrument does not have a reference beam, and the light is not modulated. The monochromator, not the filter wheel, provides the frequency discrimination to obtain a Raman spectrum.
U.S. Pat. No. 4,648,714 teaches the use of a rotating filter wheel to select Raman frequencies to detect respiratory and anesthesia gases with one detector. The filter wheel serves only to provide a selection of Raman frequencies. A black filter is provided in the filter wheel for referencing the dark signal of the detector. Neither modulation nor switching between reference and sample beams is taught or suggested.
U.S. Pat. No. 4,784,486 is a Raman spectrometer in which multiple detectors and associated interference filters are used instead of a filter wheel to select Raman frequencies. This greatly complicates the device, and would not provide the advantages of stability and low cost sought for many applications.
Some luminescence spectrometers have used filters to provide a simple instrument. U.S. Pat. No. 3,999,062 uses a circular variable filter and a light path divided into a sample and a reference beam to provide an instrument to measure the amount of fluorescence. The instrument loops the light beams back through the filter wheel for a second pass, reducing the amount of space useful for wavelength selection. The wheel is designed such that a dark signal is not obtained. The main function of the device is to alternate between polychromatic and monochromatic excitation. This complexity requires a particular design for the light paths and a particular method to analyze the signal information, but does not lend itself to the purpose of providing a simpler instrument for general emission spectroscopy.
U.S. Pat. No. 4,477,190 teaches the use of two filter wheels that are synchronously rotated to provide light to multiple samples: the resultant light outputs from the samples are directed to multiple detectors. Although this provides a multichannel spectrophotometer, it does not provide a simple spectrometer, nor does it provide these capabilities from a single moving part.
U.S. Pat. Nos. 4,945,250 and 4,977,325 teaches the use of a filter wheel for a UV fluorescence spectrometer. It uses pairs of filters, thereby reducing the available space on the filter wheel for selection. Additionally, two detectors are used, incorporating an additional source of drift. The geometry of the housing is specifically designed for this filter wheel, and does not provide for a flexible modulating, switching, and wavelength selection component for luminescence spectroscopy.
U.S. patent application Ser. No. 737,824, filed Jul. 26, 1991, teaches a forward light source spectrophotometer which has a single moving part that requires only a single light source and a single detector and provides modulation, wavelength selection, and switching between sample and reference beams.
It is therefore desirable to provide a spectrophotometer which has dual sample and reference beams, filters for a variety of bands, corrects for unwanted signal, has a minimum of moving parts, and is relatively rugged, and stable.